This invention relates to semiconductor devices, and more particularly to laser programming of such devices.
Various methods have been employed for selectively programming semiconductor devices after manufacture is essentially complete. The type of semiconductor device ultimately used is a key determining factor in deciding which programming method to use.
For semiconductors of the MOS type, it is a given that the substrate is biased at a known voltage. For microprocessors the substrate is biased at ground. A microprocessor does not have the requirement for long term charge storage in capacitors where one side of the structure is the substrate. One method for programming elements in an NMOS integrated circuit is disclosed in U.S. Pat. No. 4,387,503 to Aswell et al. Aswell teaches that semiconductor devices (NMOS circuit is shown) can be programmed by means of a laser by damaging the n moat in a p substrate, damaging the substrate under the gate of an n channel transistor or damaging the dielectric between the gate metal and the n moat of an n channel transistor. The programming method taught by Aswell can be an effective programming method as long as the substrate is biased at ground. Aswell does not, however, take into account a substrate biased at a negative potential.
For NMOS and CMOS DRAMs the substrate is held at a negative voltage. The negative voltage may be -4 volts. Damaging a junction with the substrate, as taught by Aswell, creates problems when the substrate is biased at -4 volts. The -4 volt signal is transmitted through the circuit which is typically designed to operate between ground and Vcc, typically +5 volts for CMOS and NMOS devices. The extra voltage presents difficulties for the transistors. Moreover, many CMOS devices are also designed for low power consumption. If the programming method of Aswell is used, the voltage drop is from Vcc to the substrate potential Vbb. In conventional CMOS and NMOS circuits, the substrate is biased at ground, in which case no problems should arise with laser diode programming. If, however, the substrate is biased to -4 volts, as is conventional with some DRAMs, increased voltage drops equivalent to 9 volts may be seen by the circuit transistors. Such a result is inconsistent with shrinking design size and low power consumption. The increased voltage drop creates breakdown design problems from the transistor source to drain as well as large voltage drops across the gate oxide. Both problems increase the power consumption of the device. To further complicate the problem Vbb on an NMOS or CMOS DRAM is typically produced with a substrate pump which has a limited current carrying capacity. If a junction with the substrate is damaged, the charge pump may not be able to handle the increase in leakage to the substrate. Even worse, the charge pump may be redesigned larger causing it to draw more current at all times. This is counter to the requirements of the CMOS environment where the device is to have as low a power consumption as possible.
It is the principal objective of this invention to provide an improved method of programming of semiconductor devices, particularly by laser beam make-link programmable elements. Another objective is to provide an improved method of programming MOS devices having a substrate biased at a negative voltage. Yet another objective is to provide a laser programming method which requires less space on the semiconductor substrate for implementation. A further objective is to provide a laser programming method which is less disruptive of the surrounding structure and materials, and/or which leaves a minimum of residue. Other objectives include lower dwell time needed for the leaser beam (thus faster programming) and lower power (thus less heating).